1. Field of the Invention
This invention pertains to amine catalyst for the urethane reaction.
2. Description of the Prior Art
It is known to prepare foamed polyurethanes by the reaction of a polyisocyanate, a polyhydroxyl compound and a blowing agent, such as a halogenated hydrocarbon, water, or both, in the presence of a catalyst. The catalyst is employed to promote at least two and sometimes three major reactions that must proceed simultaneously and competitively at balanced rates during the process in order to provide a good polyurethane foam. One reaction is a chain-extending isocyanate-hydroxyl reaction by which a hydroxyl-containing molecule is reacted with an isocyanate-containing molecule to form a urethane. This increases the viscosity of the mixture and provides a polyurethane-containing secondary nitrogen atoms in the urethane groups. A second reaction is a cross-linking isocyanate-urethane reaction by which an isocyanate-containing molecule reacts with a urethane group containing a secondary nitrogen atom. The third reaction is an isocyanate-water reaction by which an isocyanate-terminated molecule is extended and by which carbon dioxide is generated to blow or assist in blowing the foam. The third reaction is not essential if an extraneous blowing agent, such as a halogenated normally liquid hydrocarbon or carbon dioxide, for example, is employed, but is essential if all, or even a part, of the gas for foam generation is to be generated by this in situ reaction.
The reactions must proceed simultaneously at optimum balanced rates relative to each other in order to obtain a good foam structure. If carbon dioxide evolution is too rapid in comparison with chain extension, the foam will collapse. If chain extension is too rapid in comparison with carbon dioxide evolution, foam rise will be restricted, resulting in a high-density foam with a high percentage of poorly defined cells. The foam will not be stable in the absence of adequate cross-linking.
It has long been known that tertiary amines, such as trimethylamine, triethylamine, etc., are effective for catalyzing the cross-linking reaction. Some of the tertiary amines are effective for catalyzing the water-isocyanate reaction for carbon dioxide evolution. However, tertiary amines are only partially effective as catalysts for the first chain-extension reaction. To overcome this problem, the so-called "pre polymer" technique was developed wherein a hydroxy-containing polyol component is partially reacted with the isocyanate component in order to obtain a liquid pre polymer containing free isocyanate groups. This prepolymer is then reacted with additional polyol in the presence of a tertiary amine to provide a foam. This method is still commonly employed in preparing rigid urethane foams, but has proven less satisfactory for the production of flexible urethane foams.
For flexible foams, a one-step or "one-shot" process has been developed wherein a tertiary amine, such as triethylenediamine, is employed in conjunction with an organic tin compound. Triethylenediamine is particularly active for promoting the water-isocyanate reaction, and the tin compound is particularly active, in synergistic combination with the triethylenediamine, for promoting the chain-extension reaction. However, even here, the results obtained leave much to be desired. Triethylenediamine is a solid and must be dissolved before use to avoid processing difficulties. Also, many prior art amine catalysts, even dimethylaminoethanol, tend to impart a strong amine odor to flexible polyurethane foams, particularly immediately after foam formation.
In U.S. Pat. No. 3,786,030 the use of dialkylaminoalkylurethanes as latent catalysts for the trimerization of polyisocyanate terminated prepolymers is disclosed. The urethane is formed by the reaction of an organic isocyanate with N,N-dialkylaminoalkanols.
While N,N-dialkylaminoalkanols, such as dimethylaminoethanol, are known to be useful as catalyst of urethane formation, they suffer in that foam produced, particularly newly made flexible foam, has a strong amine odor. It is desirable to react the components of urethane formation in the presence of a catalyst which produces negligible, or no, odor but yet has sufficient catalytic activity to provide fast cream times and long rise times while producing urethane foams having good physical properties.
Surprisingly, N,N-dimethylaminopropylamine, when propoxylated with two or three mols propylene oxide, is just such a catalyst for polyurethane formation even though having an amine content substantially less than that of other amine catalyst. While N-substituted diisopropanolamines, generally preferred, have been used, after reaction with sebacic acid, to form polyurethane elastomers, the significance of the use of the compounds of this invention as catalysts for the production of urethanes, particularly polyurethane foams requiring a balance of reactions, as previously mentioned, has been totally unrecognized. (See "Synthesis of N-Substituted Diisopropanolamines, Their Sebacate Polyesters and Polyurethane Elastomers", Jean L. Boivin, Can. J. Chem. 36-1405 (1958).)